Process for plating permeable membrane



Nov. 7, 1967 c. A. LEVINE ETAL 3,351,487

PROCESS FOR PLATING PERMEABLE MEMBRANE Filed Nov. ,1963 2 Sheets-Sheet 1INVENTORS CHARLES A. LEVINE AT ORNEY Nov. 7, 1967 c. A. LEVINE ETAL3,351,487

PROCESS FOR PLATING PERMEABLE MEMBRANE Filed Nov. 6, 1963 I I 2Sheets-Sheet 2 {4 /||1 25% vb. 25 D QFA/ 2'- 2 \Q Fig.5

4 I INVENTORS CHARLES A LEVINE ffio ALFRED PREVOST ATTORNEYS UnitedStates Patent 3,351,487 PROCESS FOR PLATING PERMEABLE MEMBRANE CharlesA. Levine and Alfred L. Prevost, Concord, Calif., assignors to The DowChemical Company, Midland, Mich, a corporation of Delaware Filed Nov. 6,1963, Ser. No. 321,741 33 Claims. (Cl. 117-227) This invention relatesto an improved method of coating a permeable membrane with anelectrically conductive metallic film. More specifically, it rates to amethod suitable for coating the inner surface of a hollow fiber membranewith an electrically conductive metallic film.

The plating of metallic film on the surface of electrical- 1ynon-conducting materials such as plastic film has been desirable for anumber of purposes. One of the most important of these is for use in themanufacture of fuel cells in which electrical current is generated bythe energy given oil during the course of a controlled chemicalreaction. For such purpose it has been found desirable to have apermeable plastic membrane as the non-conducting material and to haveone or both surfaces of the membrane coated with a film of anelectrically conductive metal in such a manner that the pores of thepermeable membrane are not blocked by the metal coating.

Chemical plating of metal film on the surface of electricallynon-conducting materials has been performed by several methods asdisclosed in Patents 3,011,970 and 3,012,906. The methods of the priorart involve dipping of the surface to be plated in bulk mixtures of thedesired metal cation and a reducing agent, or co-impingement of thereagent solutions as separate sprays on the surface to be plated.Generally, in such methods the surface to be plated requirespretreatment before an adherent deposit can be made. Moreover, carefultemperature control and control'of agitation and flow rates arenecessary for uniform plating.

However, such methods as are known in the prior art are not readilyapplicable to plating of interior surfaces of small objects,particularly with regard to fine, hollow fibers. For such purposes, themethods of application known in the prior art are inappropriate,especially since the deposited metal tends to plug the interiorpassageway of the hollow fibers and also the pores of the permeablemembrane. Even when sheets of permeable membrane are plated by prior artmethods the pores become plugged by the metal deposition.

When reagent solutions are flowed through the interior of a hollowfiber, the deposit at the entrance end of the fiber builds up much morerapidly and to greater thickness than at points further advanced alongthe length of the fiber. This results in plugging one end of the fiberor depositing a thicker coat at that end. Attempts to reduce thistendency by alternately flowing the reagent solution into one end andthen the opposite end merely causes thicker deposits at both ends andthinner deposits toward the middle.

In accordance with the present invention, it has now been found possibleto take advantage of the ability of a permeable ion exchange membrane toexclude or pass certain ionic or molecular species selectively throughthe pores in the membrane. Consequently, it has now been found possibleto metal-plate the interior surface of hollow fibers made of a permeablemembrane by passing one of the reagent solutions through the interior ofthe hollow fiber and allowing the other reagent to permeate through thepores in the membrane by having the second reagent solution in contactwith the outside surface of the hollow fibers.

For example, in plating the inner surface of a hollow fiber made of apermeable membrane, the solution of an appropriate reducing agent can beflowed through the interior of the fiber and the solution of metal ionplaced in contact with the exterior surface of the fiber underappropriate conditions for permeation. through the membrane. As themetal ion passes through the pores of the membrane, it comes in contactwith the reducing agent and the metal is deposited on the interior ofthe fiber.

Since the pores in the permeable membrane are distributed uniformlythrough the length of the fiber, the resulting interior plating isdeposited in a uniform coating on the interior of the fiber.Surprisingly, however, this metal plating does not block the pores andis instead deposited on the area of the membrane between pores. Becauseof this uniformity of deposition caused by the uniformity of flowthrough out the length of the fiber, there is no need for the carefulcontrol of conditions generally required in the prior art methods evenwhere the plating is being performed on sheets of permeable membrane.

Likewise, the metal cation solution can be flowed through the interiorof the hollow fibers and the reducing agent allowed to flow from theoutside of the fibers through the pores of the membrane and, uponcontact with the metal cation solution, to cause deposition of the metalon the interior surface of the fibers. This is especially the case withanion exchange membranes. It may be desirable to complex the metalcation to make it bulky enough to prevent flow of the cation solutionthrough the pores of the membrane. It is also possible to adjust therespective pressures of the solutions inside and outside of the hollowfiber to control the flow in the direction desired. In general, reducinganions are too bulky to pass through the membrane pores. However neutralmolecular species, such as hydrazine, etc. are effective reducing agentsalso capable of permeation, if desired, through any permeable membrane.1

In addition to effecting uniform deposition as pointed out above, theprocess of this invention has the additional advantage that the initialcontact of the two reagent solutions is in close proxirniy to thesurface on which deposition is desired. In contrast, when solutions ofreducing agents and metal ions are mixed in bulk to provide dipping orspraying mixtures, the reduction or reaction which results in free metaltakes place not only near the surface to be plated but also throughoutthe mixture as a whole. This means that some of the metal can remainsuspended in the product solution or form a more porous or bulky platingwhich gives poor adherence and greater loss of metal Where flowconditions can mechanically erode such plated surface. Consequently inaddition to the greater plating uniformity, the process of thisinvention gives more efiicient use of the plating reagents by reducingthe waste, and also provides a, more dense plating with improvedadherence. The reduction of waste is particularly important in theconservation of expensive metal ions such as those of the noble metalswhich are very often used for these purposes.

Since one of the reagents enters the interior of the hollow fiber at aplurality of points uniformly distributed throughoutthe membrane, thisminimizes any localized catalytic eifect of prior deposits or conditionof the membrane. Moreover, since the flow rate of the solution goingthrough the hollow fiber does not have to be as rapid as in utilizingany prior art method Where it would be necessary to have a substantialrate of flow in order to minimize greater deposition near the entranceto the fiber interior, the flow rate of solution through the interior ofthe fiber can be very -sloW and need be only fast enough to providesufficient concentration of desired reagent and to remove reactionbyproducts therefrom. This decrease in flow rate has the additionaladvantage with the other solution immediately upon passing through thepores of the membrane, thereby causing deposition of the metal on theadjacent surface of the membrane. With membrane in sheet form, it isfound that the resulant plating has similar advantages as recited above,many of which cannot be attained by the prior art methods even thoughthere is not the additional handicap of applying such methods in smallspaces, such as the inside of hollow fibers, as described above. Forexample, even with sheet membrane the plating is much more uniform, moredense, of improved adherence, and less waste of reagents.

FIG. 1 illustrates an arrangement of equipment for plating the interiorof a single hollow fiber.

FIG. 2 is a top view of equipment suitable for plating a sheet ofpermeable membrane, and FIG. 3 is an elevational cross-sectional view ofthe same equipment.

FIG. 4 illustrates a bundle of hollow fibers which have had both endsset in a casting resin with partial crosssection of the two ends of thebundle showing how the ends of the fibers are plugged with a castingresin.

FIG. 5 shows the same bundle of FIG. 4 in which the cast resin has beencut or machined to a point where the plugged ends of the fibers havebeen cut away so as to provide free passageway through the interior ofthe fibers.

Fuel cells using gaseous reagents have been known in the art for manyyears. Such cells have certain inherent advantages over other forms ofconverting chemical energy into electrical energy. Among theseadvantages is the high efficiency of energy conversion which in mostinstances is much greater than is achieved with standard fuelconversions.

The use of a solid ion exchange membrane as the electrolyte in gaseousfuel cells has been suggested. Such membranes may be formed of cationexchange resins or anion exchange resins of various suitable materials.The use of these solid ion exchange membranes as the electrolyte ingaseous fuel cells is particularly advantageous. Since no otherelectrolyte is required, there is no problem regarding storage ofelectrolytic solution. Moreover, there is no dilution of the electrolytesince these membrane materials are solid and insoluble in water and invarious other materials with which they may come in contact.

In a particularly useful fuel cell design, the ion exchange membrane isused in the form of hollow fibers having a catalytic electrode materialcoated on the exterior surface of the hollow fiber and also a catalyticelectrode material coated on the interior surface of the fiber, with theexterior and interior coatings being electrically discontinuous witheach other. The cells are advantageously made of a plurality of suchcoated fibers with preferably at least a thousand of such coated fibersbeing employed per cell and in fact millions of such coating of thehollow fibers, the exterior coating produced by the process of thisinvention has many improvements as noted above. By reversing theposition of the respective solutions and/or the direction of solutionpermeation flow, it is possible to produce such an exterior coating.

Various methods or means can be provided for assembling such bundles andfor sealing the space between the ends of the fibers so as to provideseparate contact of the individual reagent solutions with the interioror exterior respectively of the fibers without interminglng of thesolutions. A typical method or means for such purposes is describedherein-after.

Various methods of sealing can be used. For example a suitable castingand adhesive composition can be applied to the fiber ends to fill thespace between fibers but allowed to penetrate into the hollow fibers-adistance less than the distance which they project beyond the resultingcasting. Then the projecting ends can be cut off to expose open terminalportions of the fiber.

Various means and techniques can be used for connecting the individualinterior and exterior metallic coatings in parallel electricalconductive relationship. Obviously, where there are thousands ormillions of individual fibers, it would be impractical to attempt toconnect individually the various conductors to each of the myriad ofindividual hollow fiber fuel cell elements.

In one method of making such electrical connections, the inner metallicplating of each hollow fiber extends substantially throughout the activelength of the fiber and through that portion of the fiber which extendsthrough the cast end wall or sealing means. At the 0pposite end of thisfiber bundle, the interior coating of the hollow fibers is terminatedbefore the end of the fiber and the exterior coating is extended all theway to the end of the hollow fiber.

In the operation of such a gaseous fuel cell, a fuel such as hydrogen isfed through the fibers passing through the interior of the hollow fibersand out the opposite end of the cell. An oxidizing gas such as oxygen isfed into the space between the various fibers and into contact with theexterior of said fibers. Upon permeation of one of the reactant ionspecies through the pores of the hollow ermeable fibers, the reactantscome into contact with each other and react to generate electricity as aresult of the chemical reaction. The reaction product, which in the caseof hydrogen and oxygen is water, is passed out of the system by the flowof reactant gas passing through the region in which said condensate orreaction product is formed. Means can be provided for separation andrecovery of the product, and for regeneration of the startin g reagentswhere desired.

Gaseous fuel cells in which the metal coated products of the presentinvention can be utilized are those which operate in any suitableprocess utilizing known fuel gases and oxidants. Suitable fuel gases canbe generally characterized as gaseous compounds which oxidize to give anegative free energy change (AF). Fuel gases suitable for use in suchfuel cells include hydrogen, ethylene, propylene, butene, methane,carbon monoxide, etc. While the preferred oxidant is oxygen, othersuitable oxidizing gases such as air, etc. can be utilized.

In a typical reaction wherein the membrane hollow fiber is a cationpermeable membrane, having H ions as the resultant mobile ion, usinghydrogen as a fuel gas and oxygen as the oxidizing gas, the overall cellreaction is the oxidation of hydrogen to water. The respective resultantreactions at the anode and cathode are as follows:

If the fuel cell of the present invention has the hydrogen fed into theinterior of the hollow fibers and the oxygen fed around the exteriorsthereof, then the interior surface electrode will be the anode and theexterior surface electrode will be the cathode.

While the above equations may be used to summarize the respectivereaction at the anode and cathode, it is believed that the H+ isactually passed through the membrane'in the form of H +O to react withthe oxygen at the anode, forming water. It will be seen that theformation of H +O from H+ is by the equation This reaction tends todeplete the anode side of the membrane of water.

The various ion exchange resins which are utilizable in gaseous ionexchange fuel cells all have a common characteristic of having retainedtherein water in percentages generally varying between 15 and 50%, sothat the resinous material is hydrated. This water cannot be removedfrom the resin by mechanical force, since it is retained therein bysecondary Van Der Waals forces. In order for the exchange ions to betransported across the membrane from one electrode to another, it isessential that this water be present throughout the membrane structure.By reference to the above equation it can be seen that the oxidationprocess of the cell can cause a depletion of water from the anode sideof the membrane. If water molecules are removed from the anode side ofthe membrane faster than they can diffuse back, then this anode sidewill be partially dried out, resulting in a considerably lessenedcurrent density available from the cell. The various prior art devices,relying upon thick membranes, have been subject to this process of anodemembrane drying, since their thickness is so great that the removedwater molecules cannot be adequately replaced by rediffusion of thenewly formed water molecules back to the anode side of the membrane. Inthe coated fiber structure of the present invention, the membrane wallsare sufficiently thin so that this back diffusion of water is notimpaired and proceeds at a rate sufiiciently great to prevent anodedehydration.

Assuming the fuel cell set up for gas feed as outlined above, andassuming an anion permeable membrane with hydrogen and oxygen as thefuel and oxidant gases, the overall reaction of the cell is again theoxidation of hydrogen to water with the electrode reactions at therespective anode and cathode being as follows:

It will be understood that similar reactions occur with various otherfuel gases dependent upon which ion is transported by the ion exchangemembrane.

Ion exchange resin membranes suitable for formation into hollow fibersutilizable in the gaseous fuel cells of the present invention generallyfall within three classes. The first of these classes is a hollow fiberconsisting entirely of ion exchange resin. The second of these classesconsists of a hollow fiber formed from a base resin having incorporatedtherein an ion exchange resin. The third class consists of a hollowfiber formed from a grafted base resin reacted with ion exchange formingmaterials. Any of the ion exchange resins known to the art may beutilized in the fuel cells of the present invention. Inorganic ionexchange materials are also suitable, either as such when they can bemade in permeable membranes or when embedded in a permeable membranematerial, such as zeolite in polyethylene.

As is well known, such resins contain a mobile ionic substituent. In thecase of cation exchange resins, these ions are generally attached toacidic groups such as a sulfonic acid group, a carboxyl group, and thelike. These acidic groups are attached to a polymeric material such asphenol aldehyde resins, polystyrene-divinyl benzene, polystyrene,polyethylene-grafted with styrene, sulfonated polyolefin, or otherorganic substrate. This cation component is a mobile and replaceable ionelectrostatically associated with the fused component of the resinmolecule. It is the ability of the cation to be replaced underappropriate conditions by other cations which imparts the ion exchangecharacteristics to these materials. For

suitable cationic exchange materials, reference is made to Juda'et al.Reissue 24,865, Johnson 2,658,042, Ferris 2,678,306, and Bodamer2,681,320. As preferred cationic exchange resins may be mentioned: (1)sulphonated polystyrene formed by sulfonating polystyrene or by formingan admixture of sulfonated polystyrene and other polymers, and (2)polyethylene having styrene grafted thereto by chemical or radiationmeans followed by re action with chlorosulfonic acid.

Anion exchange resin hollow fibers may be formed of any of the suitablematerials known to the art and are similar in their action to the cationexchange resins except that in the anion exchange resins it is the.ability of the anion to be replaced which causes the ion exchangeactivity. Generally speaking, such anion resins are formed byincorporating an amine group in the resin. Particularly suitable arequaternary amines. Preferred anion membranes suitable for use in thepresent invention are the following:

(l) polystyrene chloromethylated and reacted with a tertiary amine;

(2) polyethylene having incorporated therein quaternary amine ionexchange beads, such as Dowex l;

(3) polyethylene tubing having styrene grafted thereto by chemical orradiation means and reacted with chloromethyl ether, this reactionproduct being further reacted with triethyl amine.

For other suitable anion exchange resins reference is made to theabove-mentioned Juda patent, Kropa 2,663,- 702 and Bodamer 2,681,319. a

The various resinous materials discussed above may be formed into hollowfibers suitable for use in the present invention by any suitable processand apparatus known to the art, such as that shown in British Patent514,638. Depending upon the fiber-forming material employed, there maybe used melt, dry and wet spinning procedures using spinerettes of anydesign apt for the purpose or by any other techniques, such as willoccur to those who were skilled in the art. Such a process may includethe incorporation of a soluble core material in the fiber, which ifused, is dissolved out of the fiber to produce the hollow uniforminterior bore. Fibers so formed will have a continuous uniform bore aswell as uniform outer and inner diameters. It is usually expedient totake up the hollow fibers on a reel or other suitable means forcollection prior to assembling them in cells or bundles for plating bythe process of the invention; i.e., the fibers are formed as continuousfilaments which are stored and otherwise treated prior to theirformation into the desired length fibers utilized in the gaseous fuelcells.

The contemplated fibers, in order to best take advantage of their largesurface area, are formed in as small dimensions as is permissible, whichdimensions will still support an inner electrode coating and at the sametime provide an unobstructed uniform bore for the passage of gasinteriorly of the fibers. Generally speaking, such fibers should nothave an outside diameter in excess of 1000 microns. The preferred rangeof outside diameter of these fibers is between 10 and 200 microns. Theinner diameter should be so selected in the preferred fibers as to holdwithin the limits of between about /sand A; of the outside diameter thethickness of the' uniform walls of the fibers. This would correspond toa wall thickness range of between about 3 to about 66 microns. It willbe understood that the thickness of the electrode coatings on theinterior and exterior walls of the fibers will generally be less thanthe thickness of the walls of the fibers, although this is notnecessarily the case. In a preferred embodiment, these coatings are heldas a maximum to the thickness necessary to carry all the current withoutundue ohmic resistance. This is ordinarily no more than a few microns,for example no more than 2 or 3 microns.

Generally speaking, any suitable catalytic material may be used for thecatalytic electrode coatings. As suitable materials may be mentioned thefollowing: metallic silver deposited by reduction in place of silvernitrate, platinum 7 black deposited from chloroplatinic acid, metallicnickel, rhodium, palladium, iridium, copper, etc.

Reference is made again to the extremely small dimensions of the fuelcells utilizing the present invention. Such construction provides a fuelcell surface area many multiples or times greater than has hitherto beenachieved per unit volume. The extreme thinness of the membranes alsoreduces the electrical resistivity of the individual cells and insuresadequate redifiusion of the water within the membrane.

In the drawings, FIG. 1 represents an arrangement of equipment which canbe used to plate the inside of a single fiber. Container 1 has a singlefiber 2 sealed into position by sealing Composition 3 so that the metalion solution 4 can be passed through inlet 5 into the container and outthrough outlet 6 without intermingling with the reductant solution 7which is passed through the inside of the fiber. As explained hereinwhen the metal ions permeate the fiber wall and come in contact with thereductant solution inside the fiber, the metal is plated on the insideof the fiber.

FIG. 2 shows a plan or top view and FIG. 3 an elevationalcross-sectional view of a container 1 in which a membrane sheet 8 ispositioned with metal ion solution 6 on one side and reductant solution7 on the opposite side. The membrane is fastened into position withgasket 9 providing a seal between the membrane and the joined sectionsof the container which is held firmly in position by bolt 10. As one ofthe solutions permeates the membrane and comes in contact with thesolution on the opposite side the metal is plated onto the membrane.

FIG. 4 shows an arrangement of a plurality of hollow fibers 2 cast in acasting resin 11. The ends 12 of the various fibers are plugged withcasting resin although not to a very great distance from the end of thefibers. This is demonstrated by the partial sectional views at the topand bottom of the figure.

FIG. 5 shows the same fiber bundle of FIG. 4 after the cast resin 11 hasbeen machined or out along the lines 25 shown in FIG. 4. Cutting alongthese lines has removed the plugged ends 12 and leaves the fibers opento the entire portion extending through the casting resin 11 and freefor the flow of fluid therethrough.

Coatings of catalytic material applied by the process of this inventionare more uniform in thickness, density and of improved adherence ascompared to coatings applied by various other methods. Also in suchcases, potrosity of the fiber and of the metallic coating is retained toa degree sufiicient to allow passage of ions from the interior to theexterior of the coated fiber, or vice versa, depending on theapplication and the manner in which the coated fiber is to be used.

In a prefer-red embodiment of the invention, a strong solution of ametallic cation, in a solvent which will wet the membrane, is placed onthe outside of a permeable hollow fiber. A reducing solution is made toflow through the fiber. As metallic ions pass through the pores in thepermeable membrane, they are reduced by the solution flowing through theinterior of the fiber and are deposited in situ to form a porous butcontinuous coating of the metal on the interior surface of the hollowfiber.

The invention is best illustrated by the following examples. Theseexamples are intended merely for purposes of illustration and are notintended to limit in any way the scope of the invention nor the mannerin which it may be practiced. Unless specifically provided otherwise,reference to parts and percentages in the examples and throughout thespecification are to parts and percentages by weight.

EXAMPLE I Hollow permeable polyethylene fiber having an outside diameterof 190 microns and inside diameter of about 120 microns, produced bymelt spinning through an annular orifice, is chlorosulfonated withpercent chlorosulfonic acid. The treated fiber is hydrolyzed and washedwith water several times. The fiber or tubing has a capacity of 3.5meq./gm. A bundle of such fiber is cast into a bundle by having its endssealed in accordance with the technique shown in FIGS. 3 and 4 by usingan epoxy resin composition consisting of 14.7 parts of the diglycidylether of bisphenol, 6.8 parts of soya-l,3-propylene diamine, and 1.1parts of dimethylaminopropylamine. This bundle is placed in a devicesuch as shown in FIG. 5. On the outside of the fibers is placed asolution consisting of 3.5 parts AgNO in 3.0 parts of water plus enoughNH OH to dissolve the precipitate which initial-ly forms. Through theinterior of the fibers a continuous flow is maintained of a solutionmade as follows: A solution of 0.2 part AgNO and parts of water isboiled. Then 0.166 part of Rochelle salt is added and the boilingcontinued for at least another five minutes. The resultant solution isfiltered to remove any gray precipitate. The solutions are used at roomtemperature in the plating operation and the plating operation continuedfor one hour. The inside of the fibers acquires a good adherent coatingof silver.

EXAMPLE II The procedure of Example I is repeated using four bundles ofhollow fibers made of a sulfonated ion exchange membrane (made byNational Aluminate Co. and sold as Nalfilm I cation exchange membrane).Two solutions are prepared and applied to each bundle respectively inthe same manner as in Example I. After 9 minutes, one of the bundles isremoved and the fibers cut open for inspection; no noticeable deposithas been formed. After 22 minutes, another bundle is similarly inspectedand dark patches are formed on the inside surface. After 39 minutes, thethird bundle is inspected and a dull, shiny deposit is found on theinside of the fibers and very little deposited on the outside. Uponmeasuring the resistance of the respective coatings of the fibers of thethird bundle, it is found that the resistance of the inside coating isapproximately 3000 ohm/cm., and that on the outside surface has aresistance of greater than 50,000 ohms/ cm. After 59 minutes the fourthbundle is inspected and the hollow fibers have a shiny surface on theinside and a dark surface on the outside of the hollow fibers. Uponmeasuring the resistance of the respective coatings as above, theresistance of the inside coating is found to be about 5 ohms/cm., andthat of the outside coating is greater than 40,000 ohms/cm. The adherentsilver film on the inside is measured by a film micrometer and is foundto be 5-6 microns thick.

EXAMPLE III The procedure of Example I is repeated using a chloro-=sulfonated polyethylene hollow fiber having an ion exchange capacity of1.5 meq./ gm. The same solution is used externally to the fiber as inExample I but the solution passed internally through the fibers is madeof one part phenylhydrazine, 11 parts ethanol and 10 parts water. After1.5 hours, the interior surface has an adherent conducting silver filmwith a resistance of about 15 ohms/ cm. and the exterior surface has nocoating. The exterior surface has a resistance greater than 5000 ohms/cm.

EXAMPLE IV The procedure of Example III is repeated using as theexternal solution a nickel ion solution consisting of 400 grams nickelsulfate and 200 grams of citric acid per liter of solution. The solutionused on the inside of the hollow fibers consists of 8.1 grams sodiumhydroxide, 70.5 grams sodium hydrosulfide, and 10.1 grams sodiumhypophosphite. The solutions are maintained at 58 C. during the plating.At the end of 5.5 hours, the interior surface of the fibers has anadherent, smooth coat of metallic nickel with a thickness less than0.0002 inch and a resistance of approximately 15 ohms per cm. Theexterior surface of the fibers has no metallic or conductive coating.

9 EXAMPLE v EXAMPLE VI The procedure of Example V is repeated using asthe external solution one made of 10 volumes of concentrated NH OH plus4 volumes of a 30% solution of nickelous sulfate. The interior solutionis a 5% solution of sodium hydrosulfite. The plating is conducted at4550 C. for 8 hours. At the end of this period, a thin, adherent nickelmetal plating is deposited on the interior surface. This plating has aresistivity of approximately 300 ohms/ cm.

EXAMPLE VII fibers but none on the exterior surface. The copper film hasa resistivity of 70 ohms/ cm.

EXAMPLE VIII The procedure of Example VII is repeated except that theplating is conducted at 55 C. for 4 hours. The resultant interior copperfilm has a resistivity of 30 ohms/cm.

EXAMPLE IX The procedures of Examples I-VIII are repeated using in placeof the hollow fibers, a sheet of the same membrane having the samethickness as the wall thickness of the corresponding hollow fiber. Themembrane sheet is assembled in the equipment as shown in FIG. 2.. Ineach case the solution previously flowed through the interior of thehollow fibers is placed in chamber B- and the external solution isplaced in chamber A. In each case similar results are obtained as withthe hollow fibers in the respective examples.

EXAMPLE x The procedure of Example IX is repeated using as the permeablemembrane sheet a permeable membrane made of a commercial anion exchangeresin having quaternary ammonium groups attached thereto (made byNational Aluminate Co. and sold as Nalfilm II). In chamber B is placed a0.1 molar solution of silver nitrate to which sufiicient NH OH has beenadded to redissolve the precipitate which originally forms. In chamber Aa 5% aqueous solution of hydrazine hydrate is placed. After plating at14 C. for 1.25 hours, a heavy adherent metallic deposit of silver isformed on the surface in contact with the silver solution. Theelectrical resistance of the silver plating is less than 0 .5 ohms-cm.There is no deposit on the opposite side. The above procedure isrepeated with similar results with a permeable membrane made ofpolystyrene chlorornethylated and then reacted with trimethylamine.

EXAMPLE XI A fiat permeable ion exchange membrane is made by sulfonatinga polyethylene sheet with chlorosulfonic acid and hydrolyzing theproduct. On one side of this membrane is placed a platinum solutionconsisting of a mix ture of 8 milliliters of 2.67% chloroplatinic acidand 0.96 milliliters of 0.94 molar sodium hydroxide. On the other sideis placed a 4% aqueous solution of hydrazine hydrate. After standing atroom temperature for 1 /2 hours,

a thin coating of platinum is deposited on the side of the membrane incontact with the hydrazine solution.

EXAMPLE XII Hollow fibers made of sulfonated polyethylene are used asdescribed in Example I. On the outside of the fibers is placed anaqueous palladium solution consisting of .03M palladium chloride and 2Mammonium hydroxide. Inside the fibers is placed a 5% aqueous solution ofhydrazine hydrate. After standing at room temperature for one hour, theinsides of the fibers are plated with palladium metal.

EXAMPLE XIII A sulfonated nylon permeable membrane (availablecommercially from Gelman Instrument Company, of Chelsea, Mich. and soldunder the trademark Accropore No. 5A 6404 Resin) is used in sheet form.On one side is placed an aqueous solution containing 1% HAuCL; madeslightly alkaline with KOH. On the other side is placed a 1% hydrazinehydride solution in water. At the end of two hours, a gold plating hasformed on the side of the membrane in contact with the solution of gold.

Optimum conditions for the practice of this invention vary according tothe particular metal being used and the metal ion concentration.Generally, however, highly concentrated metal ion solutions arepreferred. The reducing solution, however, must be weak enough so thatthe reducing reagent contained therein will not penetrate the membranetoo fast and thereby reduce the metal ion so fast as to adversely afiectthe adherence of the metal coating. However, the reducing solution mustbe of sufficient strength to reduce the metal ion at a practical rate.Appropriate concentrations differ for different reducing agents, pHconditions, etc.

Where cation transmigration through either a cation exchange or anionexchange membrane is desired, the cation concentration should besuflicient to overcome the .ion exclusion action of the membrane. Withan anion exchange membrane, the minimum concentration of cation suitablewill be higher than with a cation exchange membrane.

In the practice of this invention where the reducing reagent is designedto transmigrate through the membrane, the cation concentration isdesirably maintained at a level which will minimize the rate of cationtransmigration through the membrane. However, it is desired to havesufficient concentration to give an adequate rate of deposition. Thisparticular embodiment is more practical with an ion exchange membranesince this type of membrane will have a much stronger exclusion actionand therefore permit a higher cation concentration thereby giving abetter deposition rate. As an alternative, the cation concentration canbe increased by using it in the form of a bulky complex in which casethe ion exclusion action of the membrane is increased and can toleratehigher concentrations of the cation without permitting transmigration.

Where an ionic reductant is used as the migrating species, similar butconverse considerations apply. If a neutral reductive is to be used, itshould be of a molecular species such that effective exclusion ortransmigration is favored according to the application desired.

However, concentrations found particularly suitable in the practice ofthis invention are 0.2% to 50% for the metallic component and 0.12% to10% for the reducing component.

Although aqueous solutions have generally been indicated herein, thesolvent can be any other solvent in which the reactants are soluble andwhich solvent does not dissolve the membrane or otherwise adverselyaffect it.

In addition to the Ag, Cu, Ni, Au, Pd and Pt shown in the examples,various other metal ions which are capable of being reduced chemicallycan also be used, such as Cr, etc. Moreover, other reducing agents suchas sodium phosphite, etc. can also be used.

Although the ionic type of permeable membranes have been illustrated inthe examples above, it is also possible to deposit a metal plating onhollow fibers or sheet membranes made of non-ionic permeable membranes.Typical of such materials are various organic polymeric materials suchas the acetate, triacetate, formate, propionate, nitrate, etc. Esters ofcellulose, including the mono-, di-, and triesters in mixtures of suchesters; cellulose ethers, such as methyl, ethyl, hydroxyalkyl,carboxyalkyl, etc., including mixed cellulose ethers; regeneratedcellulose; polyvinyl alcohols; polysaccharides; casein and itsderivatives; synthetic linear polyamides, polycarbonates, polyvinylchloride and its copolymers, polyvinylidene chloride and its copolymers,acrylic ester polymers, organic silicon polymers, polyurethanes,polyvinyl formals and butyrals, and mixtures thereof, methacrylatepolymers, styrene polymers, polyolefins such as polyethylene,polypropylene, etc., and other polyesters, and mixtures of theforegoing.

Methods of making continuous hollow fibers suitable for the practice ofthis invention are known in the art, for example see British Patent514,638. In general, such fibers are spun by melt, dry or wet spinningtechniques depending upon the particular fiber-forming materials beingused. The spinnerette is selected according to the type of spinningprocedure used and the particular dimensions desired in the hollowfiber. For the production of the hollow fiber, the spinnerette has asmall annular opening in the orifice through which the spinningcomposition is extruded.

As a typical example, cellulose triacetate is spun into continuoushollow fibers by a wet spinning process in which the cellulosetriacetate, together with whatever plasticizer or modifier is considereddesirable to impart ultimately the permeable character, is dissolved ina suitable solvent to form a viscous spinning solution. This solution isextruded through the spinnerette into a coagulant bath. As the extrudedsolution comes in contact with the bath the cellulose triacetatecoagulates or gells in the desired form of a continuously hollow fiberof uniform wall thickness. If the coagulant bath is appropriate forimparting permeability to the fiber material, this characteristic isimparted to the fiber directly. If the coagulant bath is not soconstituted, the fiber is led into a second bath to perform thisfunction. The hollow fiber is then washed free of solvent or reagentsand then either is used directly in accordance with the practice of thisinvention or is stored on a reel or bobbin or other suitable device forsubsequent use.

According to this technique, extremely fine hollow fibers can beproduced. The wall thickness is desirably sufficient to withstandpressures that may be exerted in the subsequent utilization of thesefibers. It is found that the small diameters of these fine hollow fiberspermit the self-supporting membrane walls of the fiber to withstandconsiderable pressures.

It is generally advantageous that the outside diameter of the hollowfibers does not exceed 300 microns. Preferably the outside diameters arein the range of about to about 200 microns. Advantageously, the wallthickness of the fibers is in the range of about 1 micron to about 80microns, preferably from about 2 to about microns. Wall thicknessesbelow this range may result in an inability to withstand the desiredpressures, whereas thicknesses above this range increase the resistanceto permeation through the fiber wall. Obviously, these characteristicswill vary somewhat with the particular material being used and also theparticular type of separation involved. Corresponding methods ofpreparing the permeable membrane and sheet form are well known in theart.

g 12 EXAMPLE XIV The procedure of Example I is repeated using hollowpermeable fibers of the type used in Example X. In each case an aqueoushydrosulfite solution is passed on the out side of the fibers and anaqueous nickel solution is passed through the inside of the fibers. Theaqueous hydrosulfite solution consists of 0.74 percent NaOH and 6.5percent Na S O The nickel solution consists of 26 percent nickelousacetate and 13.7 percent citric acid. The plating is conducted at 2030C. After 4 hours a nickel plating is satisfactorily effected and thethickness of the plating increases as the plating is continued for 24hours.

EXAMPLE XV The procedures of Examples I and II are each repeated twicein one case passing 0.5 percent aqueous hydrazine hydrate solutionthrough the inside of the fibers and a 0.03 molar aqueous solution ofPdCl in contact with the outside of the fibers, and in the other casepassing a solution of 4 percent hydrazine hydrate aqueous solutioninside the fibers and a 0.05 molar PdCl solution outside the fibers.After one-half hour of plating a layer of palladium is coated on theinside of the fibers. This plating is increased in thickness as theplating is continued through a period of 4 hours.

The reducing agent used in any particular plating operation is selectedas one having a reducing potential greater than the reducing potentialof the metal compound from which the metal is to be plated. Preferredreducing agents are hydrazine, hydrazine hydrate, acid salts ofhydrazine such as the sulfate, chloride, phosphate, etc., the alkalimetal borohydrides such as the sodium and potassium borohydrides,Rochelle salts, alkali metal hydrosulfites and alkali metal phosphites.With such preferred compounds, a selection is made according to which ofthese reducing agents has a greater reducing potential than theparticular metal compound from which the metal is to be plated.

The permeable membranes used in the practice of this invention can be ofa heterogeneous type in addition to the various homogeneous typesindicated above. For example, palladium ions have been passed through amembrane made by hot pressing a mixture of powdered zeolite and powderedpolyethylene as illustrated below in Example XVI. Moreover permeablemembranes can be derived by partially impregnating a pressed mat ofglass wool or other fibers such as asbestos.

EXAMPLE XVI A permeable membrane is made by mixing 30 percent by weightpowdered polyethylene and 70 percent by Weight of zeolite (Linde 4 AMolecular Sieve). This mixture is pressed at C. for 2 minutes to give amembrane thickness of 4 mils. This membrane is plated on one side withpalladium metal by the use of equipment shown in FIG. 2, using asolution of 0.07 moles PdCl and 4 moles NH OH on one side of themembrane and a solution of 4 percent N H -I-I O on the other side. Afterallowing this to stand at room temperature for 45 minutes, palladiummetal is deposited on the side of the membrane in contact with thehydrazine solution. No visible palladium is seen in the still clearhydrazine solution and the PdCl solution is also still clear.

In addition to the uses indicated above, the internally plated,permeable hollow fibers produced by this invention can be used forconducting in solution various reactions catalysed by a metal,particularly where it is necessary to minimize secondary reactionsbetween the desired product and one of the reagents. By use of thepermeable hollow fibers of this invention, one of the reactants can beintroduced to the reaction zone and into contact with the metal surfaceby permeation, or a reactive product can be removed from the reactionzone by permeation. A reactor made of such hollow fibers provides'a verylarge surface of catalyst per unit volume of reaction mixture.

not involved, the permeable membrane of this invention is usefulas'ameans for driving or promoting by removal of product a reactionwhich would otherwise slow down or stop because of equilibriumconditions attained.

While certain features of this invention have been described in detailwith respect to various embodiments thereof, it will, of course, beapparent that other modifications can be made within the sphere andscope of this invention and it is not intended to limit the invention tothe exact details shown above except insofar as they are defined in thefollowing claims:

The invention claimed is:

1. The process of plating a permeable membrane with a thin metalliccoating comprising the steps of:

(a) contacting one side of said membrane with a solution of a compoundof the metal to be plated,

(b) contacting the opposite side of the said membrane wih a solution ofa reagent capable of reducing said metal in said metal compound solutionto a metallic state,

(c) effecting permeation of said membrane by the one of said solutionswhich is on the opposite of said membrane from that On which said metalis to be plated, and

(d) maintaining said contacting and said permeation of said membrane ata temperature of to 100 C. until a sufficient thickness of metal platinghas been effected.

2. The process of claim 1 in which said solution of said reducingreagent is in contact with that side of said permeable membrane which isto be plated, and said solution of said metal compound is in contactwith the opposite side of said membrane and is permeated through saidmembrane.

3. The process of claim 1 in which said solution of said metal compoundis in contact with said side of said permeable membrane which is to beplated, and said solution of said reducing agent is in contact with theopposite side of said membrane and is permeated through said membrane.

4. The process of claim 1 in which said permeable membrane is a cationicmembrane.

5. The process of claim 1 in which said permeable membrane is an ionicmembrane.

6. The process of claim 1 in which said permeable membrane is shaped inthe form of a hollow fiber.

7. The process of claim 6 in which said permeable membrane is anon-ionic permeable membrane.

8. The process of claim 6 in which said permeable membrane is a cationicpermeable membrane.

9. The process of claim 8 in which said hollow fiber has an outerdiameter of -300 microns and a wall thickness of 1-80 microns.

10. The process of claim 9 in which said reducing solution is passedthrough the interior of said fiber and said metal compound solution iscontacted with the exterior surface of said permeable hollow fiber andpermeated into the interior region of said hollow fiber thereby todeposit a plating of metal on the interior surface of said hollow fiber.

11. The process of claim 10 in which said cation permeable membraneconsists essentially of a sulfonated polyolefin material.

12. The process of claim 10 in which said cation permeable membraneconsists essentially of an inorganic ion-exchange material.

13. The process of claim 10 in which said cationic membrane consistsessentially of an organic substrate having acidic groups.

14. The process of claim 9 in which said metal compound solution ispassed through the interior of said hollow fiber and said reducingsolution is contacted with the exterior of said hollow fiber andpermeated into the interior of said hollow fiber, thereby to deposit ametal plating on the interior surface of said hollow fiber.

15. The process of claim 14 in which said reducing agent has a reducingpotential greater than the reducing potential of the metal compound fromwhich the metal is to be plated and is selected from the classconsisting of hydrazine, hydrazine hydrate, acid salts of hydrazine,

.alkali metal borohydrides, Rochelle salts, alkali metal hydrosulfitesand alkali metal phosphites.

16. The process of claim 15 in which said metal compound solution has aconcentration in the range of 0.1-50 percent by weight.

17. The process of claim 16 in which said reducing reagent soltuion hasa concentration of 0.1-20 percent by weight.

18. The process of claim 6 in which said hollow fiber is made of ananion permeable membrane.

19. The process of claim 18 in which said hollow fiber has an outerdiameter of 10 to 300 microns and a wall thickness of 1 to microns.

20. The process of claim 19 in which said reducing solution is passedthrough the interior of said fiber and the exterior of said fiber is incontact with said metal solution which is allowed to permeatetherethrough.

21. The process of claim 20' in which said metal compound is a compoundof a metal selected from the class consisting of Ag, Au, Pt, Ni, Cu, Rh,Pd and Cr.

22. The process of claim 21 in which said reducing reagent has areducing potential greater than the reducing potential of the metalcompound from which the metal is to be plated and is selected from theclass consisting of hydrazine, hydrazine hydrate, acid salts ofhydrazine, alkali metal borohydrides, Rochelle salts, alkali metalhydrosnlfites and alkali metal phosphites.

23. The process of claim 22 in which said metal compound solution has aconcentration in the range of 0.1 to 50 percent by weight.

24. The process of claim 23 in which said reducing reagent has aconcentration in the range of 0.1 to 20 percent by weight.

25. The process of claim 20 in which said anion permeable membrane is aheterogeneous membrane consisting essentially of fine particles of ananion exchange resin uniformly dispersed in a support sheet.

26. The process of claim 20 in which said anion permeable membrane is ahomogeneous membrane consisting essentially of any organic substratehaving chemically bonded thereto anion releasing groups.

27. The process of claim 26 in which an aqueous hydrosulfite solution isin contact with the outside surface of hollow fibers made of said anionpermeable membrane and an aqueous nickel solution is passed through theinterior of said hollow fibers, said aqueous hydrosulfite solutionhaving a composition of approximately 0.74 percent by weight of NaOH and6.5 percent by weight of Na S O and said aqueous nickel solution havinga concentration of approximately 26 percent by weight of nickelousacetate and 13.7 percent by weight of citric acid.

28. The process of claim 27 in which said anion permeable membrane is amembrane having quaternary ammonium groups attached thereto.

29. The process of claim 27 in which said anion exchange membrane is apolystyrene resin having a methylene group attached to each of aplurality of aromatic nuclei in said polystyrene, and said methylenegroup having also attached thereto a trimethyl-ammoniurn chlorideradical.

30. The process of claim 20 in which said anion permeable membrane is ahydrocarbon polymer having alkyl ammonium halide radicals attachedthereto.

31. The process of claim 30 in which alkyl ammonium halide radicals aretrirnethyl ammonium chloride radicals attached to a methyl groupattached to aromatic nuclei in polystyrene.

32. The process of claim 1 in which said permeable membrane is asulfonated polyethylene and is in the form of hollow fibers.

33. The process of claim 32 in which an aqueous solution having not lessthan 0.5 percent by weight and not more than 4 percent by weight ofhydrazine hydrate therein is passed through the interior of said fibers,and an aqueous solution having not less than 0.03 molar percent and notmore than 0.05 molar percent of PdCl is passed in contact with theoutside of said fibers for a period of not less than one-half hour.

References Cited UNITED STATES PATENTS 3,228,197 11/1966 Brown et 1.136-86 ALFRED L. LEAVITT, Primary Examiner.

E. B. LIPSCOMB, III, Assistant Examiner.

1. THE PROCESS OF PLATING A PERMEABLE MEMBRANE WITH A THIN METALLICCOATING COMPRISING THE STEPS OF: (A) CONTACTING ONE SIDE OF SAIDMEMBRANE WITH A SOLUTION OF A COMPOUND OF THE METAL TO BE PLATED, (B)CONTACTING THE OPPOSITE SIDE OF THE SAID MEMBRANE WITH A SOLUTION OF AREAGENT CAPABLE OF REDUCING SAID METAL IN SAID METAL COMPOUND SOLUTIONTO A METALLIC STATE, (C) EFFECTING PERMEATION OF SAID MEMBRANE BY THEONE OF SAID SOLUTIONS WHICH IS ON THE OPPOSITE OF SAID MEMBRANE FROMTHAT ON WHICH SAID METAL IS TO BE PLATED, AND (D) MAINTAINING SAIDCONTACTING AND SAID PERMEATION OF SAID MEMBRANE AT A TEMPERATURE OF 0*TO 100* C. UNTIL A SUFFICIENT THICKNESS OF METAL PLATING HAS BEENEFFECTED.
 6. THE PROCESS OF CLAIM 1 IN WHICH SAID PERMEABLE MEMBRANE ISSHAPED IN THE FORM OF A HOLLOW FIBER.